According to tradition, for each holy day of a festival, Jews living outside of Israel celebrate two. So what happens then with the two holy days at the end of Sukkot — Shemini Atzeret and Simchat Torah — which seem to be separate holidays rather than duplicates of the same event.

The start of the New Year, the month of Tishrei, is full of holy days, among them four foundational ones: Rosh Hashanah, Yom Kippur, Sukkot and Simchat Torah-Shemini Atzeret. They are as different from one another as possible. Yet, we may also think of all four as two pairs of two. The first two -- the day of memory and...